Dyntopo workflow + Dyntopo vs. Multires?
Something to be aware of with workflows is there isn't necessarily an objectively 'better' or 'worse' workflow. There are a number of ways to get to the same (or at least very similar) end result, so the workflow you use should be the one that fits you best, and lets you work in a way you like while giving you the best results in the least amount of time.

Now to answer your questions, there is no way to revert back to your mesh once you have started to sculpt with Dyntopo. That's known as a destructive process, as it permanently changes the structure of your mesh. However, if you have just clicked the button and haven't actually started sculpting, you may be able to use the tris to quads tool in edit mode. Select all of your faces, then press Alt+J and it will try and join the triangles into quads. Depending on your mesh this may have varying levels of success.

Dyntopo vs. Multiresolution is a subjective question. There are uses for each tool, and situations where one is better than the other. I discussed this in another question here.

To answer your final question (and just for future reference you should try to limit your questions per post to just one), the workflow used on the video isn't 'necessary' but may be the best way to achieve the best results. As for convenience, if it works better for you, then do it that way. Part of creating digital art is having fun and being able to express yourself, so you don't want to have a workflow where you're constantly fighting with your tools. Find what works best and stick with it.